1. Field of the Invention
The present invention relates to a negative thermal expansion glass ceramic being able to be used for wide purposes in an energy-related field, an information communication field, an electronics field, being used as a temperature compensating member in a device containing an optical fiber, such as an optical fiber refractive index grating, a connector, or the like, and a method for producing the same.
2. Description of Related Art
The optical technique is applied to not only a field of communication systems, but also wide fields, such as precise processing techniques, medical techniques, home electronic products, or industrial electronics. With the optical technique, emitting of light, condensing of light, transmission and divergence of light, or the like are carried out by using the optical fiber.
The various devices using the optical fiber are required to have a structure which does not harm characteristics of the optical fiber itself. That is, in order to prevent changing of the optical properties, which is caused by expansion, contraction, or the like of the optical fiber by a temperature change, it is required to combine materials having a desired coefficient of thermal expansion. For example, a device using a material having a negative coefficient of thermal expansion has been proposed.
For example, Japanese Patent Laid-open No. Hei 10-90555 discloses that a material having a negative coefficient of thermal expansion, concretely, liquid crystal polymer is used at a flanged portion of ferrule made of zirconia or stainless steel having a positive coefficient of thermal expansion, in a single-core optical connector.
WO Publication No. 97/14983 discloses an optical fiber diffraction grating in which liquid crystal polymer having a negative coefficient of thermal expansion covers peripheries of an optical fiber having a positive coefficient of thermal expansion in order to prevent the expanding and contracting caused by temperature changes of the optical fiber. The disclosed liquid crystal polymer (polyesteramide) has a coefficient of thermal expansion of −1.8×10−5/° C. to −7.2×10−6/° C.
Further, Japanese Patent Laid-open No. Hei 10-96827 discloses a package in which an optical fiber being provided with a refractive index grating is mounted to a supporting member having compositions based on Zr-tungstate or Hf-tungstate having a negative coefficient of thermal expansion. Concretely, a sintered body having a coefficient of thermal expansion of −12.4×10−6/° C. is formed from ZrW2O8 powders having a coefficient of thermal expansion of −4.7 to −9.4×10−6/° C.
For various instruments or apparatus in the energy-related field, an information field, or other fields, in order to prevent occurring strain or internal stress by temperature differences, a material is required, which is able to adjust the coefficient of thermal expansion to desired values, of the devices or precision parts constituting the instruments or the apparatus, moreover, which enables satisfying dimensional precision, dimensional stability, strength, thermal stability, or the like. Further, a material is required, which is mixed with organic substances or inorganic substances, for example, an adhesive, a sealing compound, or the like used in the various devices or precision parts, which enables adjusting the coefficient of thermal expansion to desired values, of these substances. Moreover, the material is required to satisfy the dimensional precision, the dimensional stability, the strength, the thermal stability, or the like of these substances.
For these materials, because of having a large heat resistance, a small coefficient of thermal expansion, or the like, ceramics, glass ceramics, glasses, metals, or other materials have been used. However, these materials have the positive coefficient of thermal expansion, that is, the materials have a property that they expand when the temperature raises. Accordingly, these materials are not necessarily optimum materials.
Therefore, for the materials used in the various devices or mixed with the substances used in the various devices, a material which has a negative coefficient of thermal expansion to negate the positive coefficient of thermal expansion of other materials used with the materials, the organic substances or the inorganic substances is desired. That is, the material having a property of contraction when the temperature raises is desired.
For the materials having the negative coefficient of thermal expansion, generally, inorganic substances, such as β-eucryptite crystals, Li2O—Al2O3—SiO2 system ceramics containing the crystals, Li2O—Al2O3—SiO2 system glass ceramics, ZnO—Al2O3—SiO2 system glass ceramics, lead titanium, hafnium titanium, zirconium tungstate, tantalum tangstate have been known.
For example, Japanese Patent Laid-open No. Sho 63-201034 discloses a method for producing a crystallized glass (glass ceramics) having a negative coefficient of thermal expansion, wherein Al2O3 and Li2O powders within an amount of a specific range are mixed with powders of volcanic vitreous deposits, the mixture is heated and melted, thereafter processed to remove strain thereof, further reheated at a temperature of a specific range for 12 to 24 hours, and thereafter annealed to obtain the crystallized glass.
With the method, by varying conditions of the heat-treatment times and heat-treatment temperatures, the crystallized glass having the negative coefficient of thermal expansion of approximately −60×10−7/° C. is obtained, which is the largest absolute value among obtained values.
However, the various materials in the publications, having the negative coefficient of thermal expansion have various problems as follows.
In the Japanese Patent Laid-open No. Hei 10-90555 and WO Publication No. 97/14983, the liquid crystal polymer used as the negative thermal expansion material is crystalline resin, so that orientation of the crystal is high. For example, there is a problem that warping is occurred in an injection-molded product. In addition, there is a problem that values of physical properties, such as the coefficient of thermal expansion, flexural strength, modulus of elasticity, or other values differ according to directions of the liquid crystal molecules.
The ZrW2O8 or HfW2O8 used as the temperature compensating members in the Japanese Patent Laid-open No. Hei 10-96827 is not thermally stable within a wide temperature range because a phase transition occurs therein at nearly 157° C. to occur a bending in a curve of thermal expansion.
The crystallized glass disclosed in the Japanese Patent Laid-open No. Sho 63-201034 is made from the volcanic vitreous deposits, so that contents of respective components, such as alkaline metal oxides, alkaline earth oxides, and transition metal oxides expect SiO2 and Li2O, which are main components and necessary to deposit the main crystalline phase are not able to be adjusted. Accordingly, it has disadvantages that it is difficult to avoid a composition change, to deposit desired crystalline phase of a desired amount, and to produce a crystallized glass having the stable physical properties and qualities.
Further, as shown in the disclosed examples of the publication, mixed powders are melted to be cullets, the cullets are ground and melted again at 1600° C. in the producing method. Accordingly, the processes are complicated and the melting temperature of the glass is very high, so that there are problems that the production requires labors, times and costs.
Japanese Patent Laid-open No. Hei 2-208256 discloses low thermal expansion ceramics of ZnO—Al2O3—SiO2 system, wherein the main crystalline phases are β-quartz solid solution and/or zinc petalite solid solution. The ceramics, as shown in examples, have the coefficient of thermal expansion of at lowest −2.15×106/° C. (−21.5×10−7/° C.), so that the ceramics do not have a sufficiently low coefficient of thermal expansion.
Further, because these ceramics contain a large amount of ZnO component which is easy to sublime at a high temperature, it is described in the publication that when a parent glass (base glass) is formed, too long melting is not preferable. As shown in the examples in the publication, the melting time is ten minutes which is extremely short. However, in such a short time, even if the temperature is high, the SiO2 and Al2O3 components do not melt sufficiently to remain, so that it is difficult to obtain a homogeneous parent glass. Accordingly, if the heterogeneous parent glass is crystallized, the production of the homogeneous ceramics is difficult.
When the raw materials are melted, if they are melted for hours as an ordinary way, it is possible to solve the problem about the residues. In this case, however, the ZnO component sublimes to vary the composition of the parent glass, so that it is difficult to obtain the ceramics which is stably homogeneous.
Further, the melting temperature in the example is 1620° C. which is high, so that there are the same problems as the producing method disclosed in the Japanese Patent Laid-open No. Sho 63-201034.
As described above, because the earlier materials having the negative coefficient of thermal expansion have some problems, actually, they are less used in the energy-related field, the information field, the optical communication field, or other various fields.